1. Field of the Invention
The invention concerns a pneumatic powder feed, specifically for coating powder.
2. Description of the Related Art
Such a powder feed is known from the European patent documents 0 184 994 B1 and 0 337 132 A2. EP 0 184 994 B1 states that powder can be sucked from a powder container by a fluidizing and suction unit also when the container does not feature a fluidizing bottom for fluidization of the powder. The unit can be inserted through an opening in a container lid and, as the powder quantity decreases, lowered deeper into the container while being advanced axially in it. Placing the container on a vibrator facilitates the suction withdrawal of powder which has solidified in the container by storage and shipping.
The second named, EP 0 337 132 A2, states that a feed pump for the powder can in known fashion be used as an injection device and that when using such a fluidizing and suction withdrawal unit, with which the powder can be sucked directly out of a shipping container, there will be no additional, separate powder container required, but the powder can be fed directly from the shipping container to a powder processing device. The only container required is thus the shipping container of the powder which, consequently, may be a carton, a bag or a container from metal or plastic.
In the coating of articles with powder, already minimal fluctuations of the density of the powder-air flow or of the velocity of impact of the powder particles on the article being coated can lead to unsuitable coating qualities. While the aforementioned EP 0 337 132 A2 states that the powder can be fed to a processing device without an intermediate container, this may be true for powdery sand or other materials, but not for coating powder. Tests performed with known devices in the context of the invention have shown that with them powder quantities per unit of time cannot be fed with a consistency sufficient for an adequate quality of surface coatings. Furthermore, the tests performed in the framework of the invention have demonstrated that under certain operating conditions the powder merely is stirred up in uncontrolled fashion in the container, whereas a "fluidized state" as required for a suction withdrawal, that is, a condition in which the powder particles float in an air cushion in such a way that they can be withdrawn by suction, is not achieved. Moreover, it has been demonstrated in the framework of the invention that the prior devices will under certain operating conditions "fall through" the powder in the container, instead of producing at the powder surface a fluidized powder layer. This, too, renders prior devices unsuited for direct connection to a spray device, without the intermediary of an intermediate container that features a fluidizing bottom of its own for a uniform fluidization of the powder. Furthermore, prior devices involve the risk of powder overflowing out of the container. If the container is a bag, uncontrolled movements of the fluidizing and suction withdrawal device give rise to the risk of damaging the bag.